1. Field of the Invention
The present invention relates to a shielding for a transponder comprising at least a chip and an antenna structure.
2. Description of the Related Art
In many spheres of public life, RFID systems for identifying arbitrary objects have increasingly been used within the last few years. The term RFID stands for Radio-Frequency-Identification and designates an identification by means of radio waves. An RFID system always comprises two components: an evaluation device that can be implemented as a read and/or write unit, and a transponder carrying the data used for identification.
Transponders which are nowadays produced comprise a small-area chip and an antenna structure. The most common use of transponders are contactless chip cards, which are nowadays predominantly used as means of payment in the form of cheque cards, or as access examination means in the form of access tickets or corporate identification cards, the identification data in question being stored in a storage means of the chip. Contactless chip cards allow simple handling, they are robust and their susceptibility to interference is therefore low, and they offer a plurality of interesting possibilities of use.
Progress in the field of silicon technology allows extremely low-energy, passive transponders. It is, in particular, possible to realize transponders which are fed with energy from a high-frequency field (HF field) and which store data by means of this energy and re-transmit these data by means of a damping modulation. When data are transmitted in this way, the transmission bandwidth is limited to a fraction of a carrier frequency for fundamental electrotechnological reasons, the data rates realized at present being approx. 58 kbits. Further developments in the field of chip technology will presumably lead to chips which are capable of storing a data amount of approx. 1. Mbit on a chip area of approx. 1 mm2. Reading of such an amount of data will take about 18 seconds with the bit rates realized today.
In order to increase these data rates, it will be necessary to use higher carrier frequencies; this will be possible due to further developments in the field of chip technology, especially in the field of CMOS technology, since circuits having a clock rate of 1 GHz and more can be realized, not least in view of the continuous reduction in size of the structures in question.
The antenna structure of commonly used transponders is implemented either as a conductor loop or as a dipole. The implementation as a conductor loop allows inductive incoupling of the signal and offers the advantageous possibility of resonant operation. In order to achieve this, the conductor loop is connected to a capacitance so as to form an oscillating circuit, which is tuned to the operating frequency of the contactless chip card and which defines together with a coil of the evaluation device a loosely coupled transformer.
Such inductive transponders are capable of exchanging data with an evaluation device over a distance of a few centimetres to a few meters. Transponders used for this purpose are usually operated in a frequency region of a few MHz, normally at the allowed frequency of 13.56 MHz. In view of the fact that the energy required for operating the chip is received from the evaluation device via the conductor loop in a contact-free manner so that the transponder need not be provided with a voltage source of its own and will behave absolutely passive especially outside of the operating range of the evaluation device, the necessary number of windings of the conductor loop will be too high in the case of lower frequencies and the inductances will be too low in the case of higher frequencies for realizing a conductor loop of sufficient quality.
In cases in which the antenna structure is implemented as a dipole, the transponder can be used in a so-called “close-coupling system” as well as in a “long-range system”.
Close-coupling systems are RFID systems with a very short range in which the dipole of the transponder allows a purely capacitive incoupling of signals from an evaluation device, which is located at a small distance of approx. 0.1 cm to 1 cm from the transponder and which is also provided with suitable electrode surfaces. For coupling in the signal, the two dipoles are placed parallel to one another and define thus plate capacitors via which data and/or energy is/are transmitted.
In the case of long-range systems, ranges of 1 m to 10 m distance between the transponder and the evaluation device can be achieved. In long-range systems the dipole is implemented as a dipole antenna and it is operated at very high frequencies, said frequencies being approx. 2.45 GHz and 5.8 GHz in Europe at present. An evaluation device emits power which is present at the terminals of the dipole antenna of the transponder as a HF voltage and which, after having been rectified, is used for feeding the chip.
The production of extremely thin chips, which are connected to conductor loops or dipoles that are thin as well, allows the formation of extremely thin transponders, so-called Smart or RFID labels. In the case of many applications of RFID labels it will make sense to operate these labels on metal surfaces. A typical field of application is the universal identification of goods in a shopping basket in a supermarket. In spite of the logistic advantages, the universal identification will only make sense and will only be justified if all goods are labeled in this way as far as possible, i.e., also metallic objects such as tins and primarily also packets containing a metallized foil.
The mounting of a transponder, including, e.g., a high-frequency conductor loop, directly on a metallic surface is, however, not easily possible. The alternating magnetic flux through the metal surface induces eddy currents in the conductor loop which counteract the cause, i.e., the field of the conductor loop, and which therefore damp the magnetic field on the surface to such an extent that a supply of energy to and data transmission from the chip of the transponder are no longer possible.
By inserting highly permeable materials, such as ferrites, between the conductor loop and the metal surface, the formation of eddy currents can be reduced and largely avoided. A magnetically highly permeable layer between the conductor loop and the metal support will conduct the flux lines closer to the conductor loop according to its magnetic conductivity; less flux lines will penetrate into the underlying metal and, consequently, less eddy currents will be induced. This will, however, have the effect that the inductance of the conductor loop changes and that the oscillating circuit will be detuned so that the resonant frequency will become lower. Fundamentally, the self-inductance will be increased by ferromagnetic materials and decreased by non-ferromagnetic materials. The resonant frequency will change in both cases.
Shielding against high-frequency fields is a general problem that arises in the field of technology; in the case of RFID technology, this problem has, however, a special aspect: in RFID systems, the position of the electric or magnetic field which is to be shielded against or to be conducted more precisely is known because it is defined by the geometry of the antenna structure arranged in close vicinity to the metallic support.
For producing the shielding effect, ferrite films are normally used at present. These films contain ferrite particles having dimensions in the μm range, which are embedded in polymers and therefore electrically insulated from one another. In spite of the high permeability of the individual particles, an only low overall permeability of typically approx. 10 will be obtained due to the large number of “air gaps” between the particles. A permeability of 10 means that the path length of the magnetic flux lines will effectively be reduced by a factor of approx. 3; the geometric distance between the RFID label and the metal support can be reduced by this factor, with the effect remaining the same in all other respects.
Higher values can be obtained by compact magnetic conductors, e.g., layers or compact films of highly permeable metals. For suppressing the above-mentioned eddy currents, these magnetic conductors must be structured such that they suppress a flow of current in the direction of the induced electric field. Such eddy currents extract energy from the field, and this will lead, on the one hand, in a reduction of the amount of useful energy that can be transmitted and, on the other hand, in a damping of the antenna circuit with disadvantageous effects on data transmission. A similar phenomenon is known from the field of electrical engineering, e.g., when mutually insulated blade fins are provided on transformers.